1.25. Random Number Generator support

Barebox provides two types of RNG sources - PRNG and HWRNG:

  • “A pseudorandom number generator (PRNG), also known as a deterministic random bit generator (DRBG),[1] is an algorithm for generating a sequence of numbers whose properties approximate the properties of sequences of random numbers. The PRNG-generated sequence is not truly random, because it is completely determined by a relatively small set of initial values, called the PRNG’s seed (which may include truly random values). Although sequences that are closer to truly random can be generated using hardware random number generators.” Pseudorandom number generator. https://en.wikipedia.org/wiki/Pseudorandom_number_generator (2017.05.08). The PRNG used by Barebox is LCG (linear congruential generator) non cryptographically secure, so please use with caution.
  • The HWRNG framework is software that makes use of a special hardware feature on your CPU, SoC or motherboard. It can‘t provide any guarantee about cryptographic security of used HW. Please refer to vendor documentation and/or RNG certification.

1.25.1. API

/* seed the PRNG. */
void srand(unsigned int seed);

/* Fill the buffer with PRNG bits. */
void get_random_bytes(void *buf, int len);

/* Fill the buffer with bits provided by HWRNG.
 * This function may fail with a message “error: no HWRNG available!”
 * in case HWRNG is not available or HW got some runtime error.
 * If barebox is compiled with CONFIG_ALLOW_PRNG_FALLBACK,
 * then get_crypto_bytes() will print “warning: falling back to Pseudo RNG source!”
 * and use PRNG instead of returning error.
int get_crypto_bytes(void *buf, int len);

1.25.2. User interface

  • /dev/hwrng0 provides access to first available HWRNG. To examine this source you can use:
md -s /dev/hwrng0
  • /dev/prng provides access to PRNG. To examine this source you can use:
md -s /dev/prng

To seed PRNG from user space the seed - seed the PRNG is provided. For example:

seed 12345
md -s /dev/prng